In 3rd generation cellular systems, adaptive modulation and coding (AM&C) and H-ARQ schemes are being investigated for incorporation into an enhanced uplink (EU) operation designed to offer low transmission latency, higher throughput, and more efficient use of physical resources.
The AM&C scheme allows an MCS to be dynamically adjusted on a transmit time interval (TTI) basis whereby, for each TTI, the MCS is selected to make the most efficient use of radio resources and to provide the highest possible data rates. A less robust MCS uses less physical resources, but is more vulnerable to errors. A more robust MCS uses more physical resources, but offers greater protection against errors.
The H-ARQ scheme is used to generate transmissions and retransmissions with low latency. A primary aspect of the H-ARQ scheme is that data received in failed transmissions can be soft combined with successive retransmissions to increase the probability of successful reception. Either Chase Combining (CC) or incremental redundancy (IR) may be applied. When CC is applied, the same MCS is chosen for the retransmission. When IR is applied, a more robust MCS is used in each retransmission.